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Optical Differences Telescope vs Telephoto Lens?

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#1 skD1amond

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Posted 12 July 2020 - 07:47 PM

I'm new to astrophotography (and photography for that matter). I've been looking at equipment and I have a fairly basic question about mounting a camera to a telescope or using a telephoto lens. To make this a little more concrete I came up with the following example

 

Telephoto Lens Sigma 100-400mm f/5-6.3 DG DN OS

Specs: f/6.3, focal length: 400 mm, aperture: 63.5mm, price $949

 

Refractor Telescope -  William Optics Guide Star 61 APO Refractor Telescope

Specs: f/5.9, focal length: 360 mm, aperture: 61mm, price $398

 

The specs seem comparable. Well, the telephoto lens is more than twice the price but I also end up with a zoom lens that I can use for the rest of my photography. There are other differences with respect to ease of mounting, detector options, etc. but my question is about the optics. Is there anything special about telescope optics that is different than for telephoto lenses? 



#2 Michael Covington

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Posted 12 July 2020 - 08:09 PM

OK, further complicating the picture is the fact that you named a telescope designed for astrophotography.  First let's consider:

Telescope (conventional):  Designed to be as sharp as possible at the center of the field.  Designed for a fixed f-ratio.

Telephoto lens:  Designed to be reasonably sharp over a wide field.  Designed for many f-ratios (that is, it should work well at any setting of the f-stop).

 

The APO refractor that you mentioned is a little more complicated because it is (probably) designed for astrophotography (I don't know if you are expected to add a field flattener to this one).  But it is still designed to work well at only one f-ratio.  That makes it easier to achieve good optical performance.

 

The telephoto you mentioned is a zoom.  That means more compromise.  It is designed to work reasonably well at many focal lengths rather than just one.

 

Designing any lens system is a series of compromises.  No lens is perfect.  All the elements are chosen to correct each other's aberrations.  The simpler the task, the better the results.  So an excellent 360-mm f/5.9 lens is an easier problem, and for a given price will perform better, than a 100-400-mm zoom lens set to 360 mm f/5.9.

Astronomical telescope users also tend to be pickier about manufacturing tolerances.  Astronomers star-test their instruments; wildlife photographers don't.


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#3 gezak22

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Posted 12 July 2020 - 08:38 PM

There are a million differences, but in short, 'regular' camera lenses can be used for astrophotography.

 

But one major difference is that telescopes were designed for astronomy, while lenses are typically designed for general photography. I used to have a well-regarded lens, which did not give the best star shapes across the field. I inquired with the manufacturer and was quickly told that the lens was "within spec". Well, the "astrophotography spec" is higher than the normal lens "spec", so if you want to do astrophotography, get the optics that were designed for it.

 

Also, as Michael mentions above, the optical quality of typical zoom lenses is a bit below that of primes. And more specifically, zoom lenses can have zoom creep, where the focal length changes over time, which is bad when it happens over the course of an exposure.



#4 james7ca

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Posted 12 July 2020 - 08:39 PM

All the issues mentioned above by Michael (although I might disagree about the fixed f-ratios) and one other very important difference. A telescope is designed to be used when focused at infinity, while a camera lens has to perform acceptably over a range of distances. As an example, a large and "fast" Newtonian reflector can be sharp and diffraction limited when focused at infinity but will suffer significantly from spherical aberration when focused on anything that is closer than several hundred yards. Refracting telescopes also don't function very well when focused very closely, but in this case the problems may only become apparent when the object is dozens of feet away.

 

Plus, telephoto lenses generally aren't diffraction limited until they are stopped down by two or three f-stops


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#5 OldManSky

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Posted 12 July 2020 - 08:59 PM

Michael gave a good explanation (note:  a flattener is suggested for the ZS61).

 

Here's my simple one:

 

APO/ED Telescope:  very good at one thing:  astronomical viewing/imaging.  Very little false color (chromatic aberration), very sharp, 2-3 lens elements specifically designed for one purpose.

 

Zoom Lens: very good at nothing.  Moderately good at a variety of focal lengths, almost always sharper/less CA in the middle than the outsides of the frame.  Only gets to "good" (instead of moderately good) at a couple of focal lengths and stopped way, way down.  Jack of all trades, master of none.

 

That's the difference :)


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#6 BQ Octantis

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Posted 13 July 2020 - 03:49 AM

I do extensive AP with a fast (f/2.8) telephoto lens. But the important distinction is that I use a prime telephoto. What you are proposing is a zoom lens. Prime telephotos are always better than the zooms that encompass them—usually much, much better. Zooms have to make way more compromises than a prime—with the cost being poorer star shapes, more CA, less color accuracy, more corner distortion, and poorer image quality.

 

I would not recommend the proposed zoom for AP.

 

BQ


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#7 Hesiod

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Posted 13 July 2020 - 05:02 AM

In my opinion the best asset of lenses is, beside the obvious fact that the DSLR is designed to handle them so would keep the same feeling you are used to, their speed.
IMHO going to f/2.8-f/2-f/1.4 is really huge on certain targets, and well worth the compromise about star shapes (by the way, in PP can circumvent this issue to a good extent).
There are also few (mostly expensive) telescopes capable of such speed, but are much more harder to tame.
As for the zooms, in my limited experience are not as good as "primes" but the more recent ones could be quite good.
I have recently got an EOS R and that was bundled with a 24-105 f/4 zoom which does not seem the usual "crap" stock lens.
Next WE will give it a try under a dark sky, but from shots taken from home seems somewhat viable for the job
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#8 nimitz69

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Posted 13 July 2020 - 10:36 AM

The answer to your question is: yes, they are different

Having said that, as has been mentioned, you can do AP with DSLR camera lens, the best being fast prime lens but a telescope will always perform better for AP

The right tool for the right job.

Ever play in one of those ‘fun’ golf tournaments where they only allowed you to have 2 clubs to play the entire 18 holes? It’s quite entertaining and you are technically still playing golf but making your normal handicap will likely be somewhat of a challenge ...

Edited by nimitz69, 13 July 2020 - 01:49 PM.

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